Reusable medical instruments and pharmaceutical and biological equipment are generally sterilized before each use. Additionally, reusable containers employed in medical, pharmaceutical, and biological applications, such as gloveboxes and incubators, are generally sterilized before each use. Containers such as cassettes are employed first for sterilizing articles and then for maintaining the sterility of articles during post-sterilization storage. In facilities and applications where these types of instruments and containers are used several times a day, it is important to achieve sterilization efficiently and economically.
Several different methods have been developed for delivering a vapor phase sterilant to an enclosure or chamber for sterilizing the load (e.g., medical instruments or other articles) or interior thereof. In one option, the "deep vacuum" approach, a deep vacuum is used to pull liquid sterilant into a heated vaporizer; once vaporized, the sterilant vapor is drawn into an evacuated and sealed chamber. In another option, the "flow-through" approach, vaporized sterilant is mixed with a flow of carrier gas that serves to deliver the sterilant vapor into, through and out of the chamber, which may be at slightly negative or positive pressure.
Bier, U.S. Pat. No. Re. 33,007, Aug. 1, 1989, incorporated herein by reference, discloses a method of vaporizing a multicomponent liquid, such as hydrogen peroxide and water, and passing the vapor in successive small increments into a sterilization chamber.
Methods have been developed for optimizing vapor phase sterilization in a deep vacuum and/or flow-through system. Cummings, et al., U.S. Pat. No. 4,956,145, Sep. 11, 1990, incorporated herein by reference, discloses a deep vacuum method of vapor phase sterilization in which a predetermined concentration of hydrogen peroxide sterilant vapor is maintained in an evacuated, sealed chamber. The amount of liquid sterilant injected into a vaporizer is regulated or adjusted to account for the estimated decomposition of hydrogen peroxide sterilant vapor into water and oxygen in the closed system over time. In a different approach, a predetermined percent saturation is maintained in an open, flow-through sterilization system as disclosed in commonly assigned, copending application U.S. Ser. No. 08/237,406, entitled "Optimum Hydrogen Peroxide Vapor Sterilization Method," filed on May 2, 1994, and now U.S. Pat. No. 5,445,792 issued Aug. 29, 1995, incorporated herein by reference. This patent discloses regulation or adjustment of the rate of hydrogen peroxide vapor injection into a carrier gas in response to predetermined characteristics of the carrier gas.
Also, several systems and apparatus have been developed for conducting vapor phase sterilization. An open flow-through system designed to handle the disposition of residual sterilant vapors is disclosed in Cummings, et al., U.S. Pat. No. 4,909,999, Mar. 20, 1990, incorporated herein by reference. That system can be integrally associated with or releasably connected to a sealable container.
Childers, U.S. Pat. No. 5,173,258, Dec. 22, 1992, incorporated herein by reference, discloses another flow-through system in which vapor phase hydrogen peroxide is introduced into a recirculating, closed-loop flow of carrier gas. The hydrogen peroxide vapor is introduced and maintained at a predetermined concentration selected to optimize the sterilization cycle. The system includes a dryer to dehumidify the recirculating flow, preferably to at least about 10% relative humidity, and thereby prevent moisture build-up resulting from the decomposition of hydrogen peroxide vapor over time. By eliminating moisture build-up, the system can maintain the sterilization chamber at higher concentrations of vapor phase hydrogen peroxide sterilant for longer periods of time (i.e., the predried gas will accept more of the sterilant vapor). Further, to avoid condensation of the sterilant, the relative humidity in the chamber is preferably reduced (e.g., to at least about 10%) prior to introducing the sterilant vapor. After decontamination is complete, the enclosure may be rehumidified or conditioned if desired for the selected application.
Gas sterilization/decontamination systems rely on maintaining certain process parameters in order to achieve a target sterility or decontamination assurance level. For hydrogen peroxide gas sterilization/decontamination systems, those parameters include the concentration of the hydrogen peroxide vapor. By maintaining a sufficient concentration of hydrogen peroxide vapor and/or percent saturation at various temperatures and pressures for a sufficient period of time, desired sterility assurance levels can be successfully obtained while avoiding condensation due to vapor saturation. Existing systems typically monitor the amount of liquid delivered to the vaporization system over time, and, based on temperature, pressure, volume, and (where applicable) flow rate, calculate the theoretical concentration of hydrogen peroxide vapor, and then correlate some or all of these parameters with empirically derived estimates of hydrogen peroxide decomposition, to arrive at an estimate of the amount of hydrogen peroxide to inject into the system in order to maintain a sought theoretical concentration of hydrogen peroxide vapor. The sterilization performance is then validated empirically via microbiological efficacy testing.
Cummings, U.S. Pat. No. 4,843,867, Jul. 4, 1989, incorporated herein by reference, discloses a system for monitoring and controlling the concentration of one or more selected components in a multicomponent vapor, by measuring a property of the multicomponent vapor, such as dew point, measuring another property of the one or more selected components of the multicomponent vapor, such as relative humidity, and fitting the measured values for these properties into a model, thereby obtaining an estimate of the concentration of the selected component. The estimated concentration of the selected component allows Cummings to more closely control input of that component and thereby obtain a greater measure of control over its concentration in the sterilization chamber than was previously available. Cummings' method is an indirect approximation based on a number of empirical assumptions, including the estimated rate of loss of the component from the multicomponent vapor.
In actual practice several factors can affect the concentration of components of the vapor, such as decomposition, absorption and adsorption, all due to contact of the gas with various surfaces in the system, and dilution due to evaporation by water vapor from the loads being processed and to decomposition of the sterilant. These effects can vary from load to load and system to system. The need exists to adjust the supply of sterilant vapor to take these effects into account with a precise, real-time measure of the concentration of the sterilant vapor component of the multicomponent vapor in the sterilization chamber.
The foregoing methods and systems are effective at sterilization and/or provide an enhanced sterilization cycle. There exists, however, a need for further improvement in the measurement and control of the concentration of hydrogen peroxide vapor in the sterilization chamber.